1. Field of the Invention
The present invention relates to a rear projection image display apparatus which enlarges and projects an image displayed on an image display device, for example, on a transmission screen through a projection lens system.
2. Description of the Related Art
As an image source for a rear projection image display apparatus (hereinafter sometimes called a “set”), transmission or reflection liquid crystal panels or pixel-selective image display devices such as micromirror devices may be used in place of three CRTs (CRTs which display red, blue and green images). As projection lens systems for the compactness of a set which uses such image display devices, those described in JP-A No. 134213/1993 (literature 1), JP-A No. 162544/2000 (literature 2) and JP-A No. 357768/2002 (literature 3) have been widely known. These patent literature documents disclose projection lens systems which enlarge and project an image on a screen obliquely.
As described in literature 1 to 3, when an image is projected on a screen obliquely, the image projected on the screen is distorted into a trapezoidal image. In the projection optical system described in literature 1, an afocal converter located on the screen side is decentered or eccentric to suppress image trapezoidal distortion. However, the magnification of the disclosed afocal converter is too low to permit a wide view angle (make the set compact). In the projection optical system described in literature 2, it is impossible to make the view angle wide enough to realize a sufficiently thin (low-profile) rear projection image display apparatus; therefore, the lenses in use must be decentered individually and thus there is difficulty in its manufacture. The projection optical system described in literature 3 consists of a first refractor system with positive refractive power, a second refractor system with negative refractive power and an optical path turning mirror where at least two lenses in the second refractor system with negative refractive power are different in rotational symmetry or eccentric. Hence, the problem is that in the manufacturing process, it is not easy to assure positional accuracy of each lens.
In the above prior art, attention has been paid only to the projection optical system including a projection lens but few efforts have been made to optimize the overall system design in consideration of space for drive circuitry in the housing. Particularly, one problem is that the reflection mirror provided for the sake of the compactness of the set is very sensitive and its positional error might cause image trapezoidal distortion. The use of the reflection mirror somewhat helps reduce the size of the set but makes it impossible to taper the set from its front side to its back side; as a consequence, when the set is viewed sideways, it does not look thin enough.
Hence, in order to assure high quality of an image on the transmission screen and compactness of the set, it is necessary to employ an adequate housing structure and a projection optical system which provides a wide view angle, high focus, high magnification and long back focus. It is also desirable that an image from the projection lens system be directly projected on the transmission screen in enlarged form without the use of a reflection mirror.